i) Field of the Invention
This invention relates to a process and apparatus for recycling waste cellulosic material including paperboard to produce a cellulose fiber pulp, and to a cellulose fiber pulp so produced; the invention is especially concerned with recycling waste corrugated paperboard.
ii) Description of Prior Art
Paperboard, including corrugated paper-board, is widely used in packaging; millions of tons of used paperboard are discarded annually, and more than 70% of such discarded waste in Canada goes to landfill sites.
Processes have been proposed to recycle at least a portion of such waste paperboard primarily for use with virgin pulp. Prior processes involve cleaning to remove non-wood contaminants and re-pulping to form a brownstock which is then mixed with virgin pulp for making linerboard or corrugating medium for use in corrugated paperboard.
The quality of the re-pulped fibers is lower than that of comparable virgin pulp fibers and thus it is usual to use the re-pulped fibers only in admixture with higher quality virgin fibers, or to employ chemical additives such as resin or starch to compensate for the inferior quality of the re-pulped fibers. The inferior quality of re-pulped fibers as compared with comparable virgin pulp fibers arises from the processing to which the fibers were subjected during the original paperboard manufacture, and also to aging in the paper product.
In U.S. Pat. No. 4,737,238 recycling of waste paper containing aluminum is described in which an aqueous pulp suspension of the waste paper is screened and then dewatered to produce a white water stream containing most of the aluminum, whereafter the pulp is subjected to oxygen delignification. In Canadian Patent 1,110,411 waste paperboard which contains wax and resin is re-pulped by immersing it in a weak sulphate black liquor at a temperature of 150.degree. to 200.degree. F. to produce defiberization of the waste and separation of wax and resins from the fibers.
Paperboard is itself typically produced from virgin pulp. The production of virgin pulp involves reacting or cooking wood chips with an alkaline cooking liquor at an elevated temperature. The cooking liquor may be a kraft, soda, alkali, sulfite or polysulfide cooking liquor, or a modification thereof, for example an anthraquinone modified cooking liquor. The pulp yield depends on the pulping conditions including reaction time, temperature, liquor type and chemical dosage, and can be as low as 50% to as high as 85%.
Lignin is a component of the woodchips which is dissolved by the cooking liquor in the manufacture of cellulose pulp. The character of the pulp produced is dependent on the extent of lignin removal from the woodchips, and hence on the residual lignin content of the final pulp.
The kappa number represents a measure of residual lignin content. Higher kappa numbers indicate higher residual lignin levels.
The kappa number of a brownstock pulp obtained from cooking softwood in a kraft liquor is typically 50 to 100, and such a pulp is used for making linerboard of corrugated paperboard. The kappa number of a brownstock pulp obtained from cooking hardwood in a kraft liquor is typically 130 to 160, and such a pulp is used for making the corrugated medium of corrugated paperboard.
The kappa number of a hardwood brownstock pulp would need to be reduced to about 10 to 15, and that of a softwood brownstock to about 25 to 35 to provide a pulp suitable for bleaching to produce white paper products.
Corrugated paperboard waste comprising linerboard and corrugating medium has a kappa number of 80 to 120.
Reject pulp from pulping processes and other waste paper products such as old newsprint and used cardboard also represent a similar disposal problem.